Machines for packaging pourable food products—such as fruit juice, wine, tomato sauce, pasteurized or long-storage (UHT) milk, etc.—are known, on which the packages are formed from a continuous tube of packaging material defined by a longitudinally sealed web.
To produce the packages, the tube of packaging material is filled continuously with the pourable food product, and is then fed to a form-and-(transverse) seal unit on which the tube is gripped between pairs of jaws and sealed transversely to form pillow packs.
Once sealing is completed, a knife cuts the tube of packaging material along the center of the sealed portion to cut a pillow pack off the bottom end of the tube of packaging material. The bottom end being sealed transversely, the jaws, on reaching the bottom dead-center position, can be opened to avoid interfering with the top portion of the tube; and, at the same time, the other pair of jaws, operated in the same way, moves down from the top dead-center position and repeats the same gripping/forming, sealing and cutting operations.
One problem with known form-and-seal units has to do with the so-called “decoration correction” system.
That is, the web of packaging material normally comprises a series of equally spaced printed images or decorations on the portions eventually forming the outer surfaces of the packs, so that the web must be fed to the form-and-seal unit in such a manner as to register forming, sealing and cutting of the packs with the succession of decorations. In actual use, since the decorations are printed equally spaced, the position of each with respect to the position of the jaws on the form-and-seal unit may vary, firstly as a result of varying deformation of the packaging material by the mechanical pressure exerted on it by the jaws, and, secondly, as a result of the pulsating pressure of the pourable food product inside the tube of packaging material. A system for position correcting the decoration is therefore required.
On modern packaging machines, such a system comprises an optical sensor for detecting the position of a bar code on each pack; and a control unit for comparing the detected position with respect to a theoretical position.
On some commercial machines, each pair of jaws has a pair of traction members for drawing the tube of packaging material, which are movable with respect to the jaws to form triangular tabs at the top and bottom corners of the pillow packs. On detecting a decoration position error, the control unit adjusts the speed of the motor controlling feed of the web of packaging material. If this correction is not sufficient, the tube traction members are controlled to slightly increase or reduce pull on the packaging material. According to other solutions, the control unit acts directly on the tube traction members, with no possibility of adjusting the speed of the motor controlling feed of the web of packaging material; and the operation is repeated until the position of the decoration coincides with the theoretical position, which may only occur after a certain number of packs have been produced, and which must therefore be rejected. At times, this method also fails to correct the position of the decoration, as, for example, when loading a new reel of packaging material with a different decoration spacing. In which case, the machine must be stopped and reset manually to the new spacing.
European Patent Application EP-A-0 959 007 describes a form-and-seal unit of the above type, in which the reciprocating movement of each jaw is controlled by two rods activated by respective servomotors. Independent control of the four rods therefore provides for taking into account any error in the position of the decoration, and for controlling the operating speed of the jaw assemblies accordingly.